This invention relates to a method of making diffusion bonded structure.
In the past several decades, there have been substantial advances, particularly in the aerospace industry, in the manufacture of high performance structures (i.e. those structures having a high strength to weight ratio). The two general methods of joining structural components to make a composite structure are: (a) using fasteners, and (b) bonding.
The use of fasteners usually requires the forming of the holes in the structural components to be joined. However, not only do the fasteners themselves add weight, but the structure must be made somewhat larger to make up for the reduction of load carrying capacity because of the presence of the holes. Further, the drilling of holes in load carrying structure generally causes a concentration of stresses in the structure adjacent the hole, thus encouraging the growth of fatigue cracks in the structure.
For these reasons, there has been greater effort to eliminate the use of fasteners wherever possible and bond the components together (e.g. by adhesive bonding, brazing, welding, diffusion bonding, etc.). However, attempts to bond larger or more complex structural components run into various technical problems, such as obtaining proper engagement of the bonding surfaces, proper distribution of the bond material, applying adequate heat and/or pressure at the bond areas, preventing warping of the structure by application of heat, preventing embrittlement of the structure being bonded, etc.
In some applications, such as in supersonic aircraft, this general problem area has been further complicated because of the necessity of the structure withstanding sustained higher temperatures, in the order of perhaps 500.degree. F. or more. Since most organic adhesives will not stand up under such temperatures, the use of braze metals to join components together appears to offer a more attractive alternative. However, the use of such braze metals presents problems of its own. If the braze metal in the structural assembly is maintained at a melting temperature for too long a period, the braze metal may tend to flow away from the braze area. Also, there is a tendency for the braze metal to dissolve into the metal to form an embrittled alloy which is undesirable in the final structure. For these reasons, the structural assembly to be brazed must be brought up to the brazing temperature for a short period and then down below the brazing temperature. If some of the structural components are of substantially thicker cross section than others, so that these thicker components heat up and cool down at a slower rate, while some of the components of substantially lesser cross section heat up and cool down quite rapidly, attempting to bond such dissimilar components into a composite structure becomes quite problematical.
Interrelated with these above problems are those that involve the use of honeycomb which has found wide application in the aerospace industry primarily because of the excellent strength to weight ratios obtained thereby. Most commonly in structural configurations, a honeycomb core or layer is bonded to upper and lower face sheets to form a structural panel. Mainly because of the various problems in bonding such as those listed above, the honeycomb panel or panels are joined to one another or to a heavier load carrying member by the use of doubler sheets or splice plates along with fasteners. To the best knowledge of the applicant herein, the practice of using solely bonding to form a composite structure made up of both honeycomb panel and heavier structural components has not become a widely used manufacturing technique in the aerospace industry.
In more recent years, the use of titanium for aerospace structures has been investigated more intensely, primarily because of its greater strength to weight ratio and its ability to withstand higher temperatures. However, some of the above-mentioned problem areas become more critical with titanium. For example, titanium is quite susceptible to embrittlement (e.g. by gases such as hydrogen and oxygen being dissolved into the titanium when it is exposed to higher temperatures).
However, titanium does offer one promising avenue in the joining of structural components in that it is capable of being diffusion bonded. While the precise mechanism of diffusion bonding is not completely understood, it has been described as bringing the surfaces of two components into contact with sufficient heat and pressure so that the atoms adjacent the adjoining surfaces move together to make atomically or molecularly mating surfaces which hold the components together as a unitary structure. Diffusion bonding is not possible with aluminum, since aluminum components are coated with a layer of aluminum oxide which remains on the surface to prevent diffusion bonding. However, with titanium, although there is a surface coating of titanium oxide, this titanium oxide dissolves into the titanium structure at the diffusion bonding temperature so that titanium atoms from the two bonding surfaces can come into intimate contact with one another to accomplish the diffusion bonding.
A solution to a particular problem in diffusion bonding is disclosed in U.S. Pat. No. 3,633,267, issued Jan. 11, 1972, one of the inventors of which is the applicant herein. This method discloses a method of diffusion bonding metallic honeycomb structure to a face sheet. A layer of honeycomb is placed in a furnace, with a face sheet spaced above the honeycomb core by means of heat yieldable spacers. Compressive force is applied to the upper face sheet through a yieldable medium (such as glass). Upon bringing the furnace up to diffusion bonding temperature, the heat yieldable spacers permit the sheet to come into contact with the honeycomb core, and the compressive force applied through the yieldable media insures that all portions of the face sheet are in proper diffusion bonding contact with the honeycomb.
With regard to the broader aspects of diffusion bonding, substantial background information of what is known in the prior art about the mechanisms involved in diffusion bonding is provided in two articles authored by W. H. King and W. A. Owczarski, the first article being entitled, "Diffusion Welding of Commercially Pure Titanium", appearing in the "Welding Journal Research Supplement", July, 1967, and the second article entitled, "Additional Studies on the Diffusion Welding of Titanium", appearing in the October, 1968 issue of the same magazine. (In both of these articles, the authors use the term "diffusion welding" in preference to the term "diffusion bonding" to describe this particular phenomenon.)
In the earlier article the authors describe diffusion welding (i.e. diffusion bonding) as follows:
"Diffusion welding is the process by which metals are joined by causing the coalescence of the mating surfaces through the application of pressure at elevated temperature; this is usually done in a protective atmosphere. The temperatures used are substantially below the melting point of the lowest melting base metal in the assembly but high enough to permit diffusional processes to occur. The applied pressure is sufficient to assure intimate interfacial contact, but it is not sufficient for macroscopic deformation of the parts."
In the experimental work described in this earlier article, pure titanium was used. The flat surfaces of two titanium pieces were pressed against one another (at pressures of 500 pounds per square inch and 1000 pounds per square inch) and these were subjected to temperatures between 1400.degree. F. and 1600.degree. F. In the latter part of this earlier article, under the title "General Discussion", the authors further describe the process as follows:
"The joining process is described in three stages, and the mechanisms which operate during the stages are considered. The initial stage involves the deformation of the contacting surface asperities to produce intimate contact in a large fraction of the interfacial area. Intimate contact is not completely achieved, and consequently voids remain in the interface which are of various sizes and shapes. During the second stage, diffusional processes become more significant than deformation, and many of the interfacial voids are eliminated by the diffusion of atoms to the void. Simultaneously, the interfacial grain boundary migrates out of the plane of the interface toward an equilibrium configuration which is nonplanar and indistinguishable from the other grain boundaries. Some of the voids are engulfed within the grains that cross the interface. After the migration of the interfacial grain boundary is essentially complete, the final stage of the process is the elimination of the voids within the grains."
In the later article, the authors discuss experimental results of the effects of pressure on the diffusion welding (bonding) process. Their experimental results indicated that it was necessary to apply pressure during the initial stage of the bonding process (presumably in the order of 500 to 1000 pounds per square inch), and that such pressure was not necessary for the second and third stage. In discussing the mechanism by which the voids are eliminated in the second and third stage, it is stated that it is by diffusion of the titanium into the voids. In the later part of this article, under the heading "Discussion of Results", the authors also discuss the possibility of whether the void elimination in the second and third stages may be due in part not only to diffusional processes, but also to a deformation process related to the "surface tension" due to the curvature of the void surface. However, after analyzing the manner in which such "surface tension" deformation should occur if it were an influence, the authors conclude that it is very doubtful that this mechanism of "surface tension" causing elimination of the voids actually does operate.
As a general comment on the state of the art of diffusion bonding, to the best knowledge of the applicant herein, diffusion bonding has not as yet achieved any great acceptance as a practical commercial process in fabricating titanium structure or the like. Particularly, it has been quite problematical to use diffusion bond in a structure where the components comprise fragile core (such as honeycomb) as well as more substantial load carrying members.